The present invention concerns an improved process for the preparation of ultrafine high purity boron carbide powders, and its product, useful as a relatively high cost refractory material in the manufacture of ceramic parts.
A significant impediment to the increased use of ceramic materials in certain applications is the high incidence of failures in engineered ceramic parts. These failures can often be attributed to small cracks or voids in such parts, which result from incomplete packing of the precursor powders. An obvious solution to this problem is the manufacture of ultrafine monodispersed powders which can be packed tightly, thereby reducing the void spaces between particles.
Current efforts in ceramic technology are directed toward the manufacture of ceramic parts &hat exhibit the desirable physical properties of the material, e.g., hardness, maintenance of structural integrity at high temperatures, and chemical inertness, with the elimination of impurities and defects which often result in failure of the ceramic. It has been suggested, E.A. Barringer and H. K. Bowen, "Formation, Packing and Sintering of Monodispersed TiO.sub.2 Powders," J. Amer. Ceram. Soc. Vol. 65, C-199 (1982), that an "ideal" ceramic powder for producing a high quality part must be of high purity and contain particles which are monodispersed, spherical, nonagglomerated and of a fine size (0.1-1.0 micron).
As a ceramic powder is sintered, adjacent particles fuse into grains. In general, the grain size is governed by the particle size of the powder from which the part is prepared. In other words, the grain size is necessarily larger than the crystallites from which a part is sintered. Thus, the sintering of finer particles presents the opportunity to produce fine-grained bodies.
The effect of grain size on the integrity of boron carbide bodies has been investigated by A. D. Osipov, I. T. Ostapenko, V. V. Slezov, R. V. Tarasov, V. P. Podtykan and N. F. Kartsev, "Effect of Porosity and Grain Size on the Mechanical Properties of Hot-Pressed Boron Carbide," Sov. Powder Metall. Met. Ceram. (Engl. Transl.) 21(1), 55-8 (1982). The authors found that parts exhibiting a fine grain size were significantly stronger than parts consisting of coarse grains.
An additional advantage in the use of ceramic powders with a small average particle size is that the temperatures required to sinter the powders are often reduced. In their work on sintering TiO.sub.2 powders, Barringer and Bowen found that the sintering temperature could be reduced from 1,300.degree.-1,400.degree. C. to 800.degree. C. when using 0.08 micron sized particles. On an industrial scale, this could result in a considerable savings both in material and energy costs.
The gas-phase synthesis of boron carbide powders typically involves the reaction of a boron halide with a hydrocarbon as the carbon source in the presence of hydrogen. I. M. MacKinnon and B. G. Reuben, "The Synthesis of Boron Carbide in an RF Plasma," J. Electrochem. Soc. 122(6), 806 (1975), utilize a radio-frequency induced argon plasma to heat a stream of boron trichloride, methane and hydrogen. The boron carbide powders formed are about 200-300 .ANG. in diameter. British Patent No. 1,069,78 and U.S. Pat. No. 3,340,020 describe the reaction of boron trichloride-methane mixtures in a hydrogen plasma jet to produce boron carbide powders of 200 .ANG. average particle size.
The synthesis of ceramic powders using a carbon dioxide laser was first developed by Haggerty and co-workers. In their article, "Synthesis and Characteristics of Ceramic Powders Made from Laser-Heated Gases," Ceram. Eng. Sci. Proc. Vol. 3, 31 (1982), R. A. Marra and J. S. Haggerty describe the preparation of silicon, silicon carbide and silicon nitride powder from SiH.sub.4. The powders produced were quite small, equiaxed, and monodispersed with particle sizes in the range of 100-1,000 .ANG.. Their paper also contains the statement that this laser-heated process can be used to produce other nonoxide ceramics such as TiB.sub.2, AlN and B.sub.4 C as well as many oxide ceramics. However, there is no specific teaching regarding the actual production of B.sub.4 C using a laser.
The printed version of a talk given at Electro Optics/Laser International, Japan, 1984, Feb. 16-18, 1984, by J. T. Yardley and A. Gupta, entitled "Production of Light Olefins from Synthesis Gas Using Catalysts Prepared by Laser Pyrolysis," reports the successful preparation of boron carbide from boron trichloride, hydrogen and ethylene using a continuous wave CO.sub.2 laser.
One example in the literature, a technical report from the U. S. Army Missile Command, government access number AD-A101035, investigated the possibility that a CO.sub.2 laser could be used as an energy source for the reaction of boron trichloride and a hydrocarbon to produce boron carbide. However, the investigation did not produce boron carbide (B.sub.4 C).
Unexpectedly, it has now been found that a CO.sub.2 laser is an acceptable energy source for the reaction of boron trichloride, hydrogen and a hydrocarbon for the production of high purity boron carbide but only under certain reaction conditions.
Several commercial uses are available for ultrafine high purity boron carbide powders and ceramics made from such powders, for example, sand blasting nozzles, seals, abrasive powder for grinding/machining applications, and as sintering aids.
The present invention provides a process for the preparation of high purity ultrafine boron carbide powder. Also, the present process produces relatively monodispersed ultrahigh purity boron carbide powders.
According to this invention, ultrafine high purity boron carbide powder is produced by subjecting a continuous stream of reactant gases consisting essentially of a volatile boron source, less than the stoichiometric amount, calculated on the boron in the boron source, of a volatile carbon source and at least a stoichiometric amount, calculated on the boron in the boron source, of a source of hydrogen, at an absolute pressure of at least about 300 Torr, to an amount of CO.sub.2 laser radiation effective to convert at least a portion of the volatile boron source to B.sub.4 C.
This invention also concerns the boron carbide powder product prepared by the present process. The product (B.sub.4 C) powder has several unique properties compared to known B.sub.4 C powders. For example, the present B.sub.4 C powder can be hot-pressed to parts of theoretical density at temperatures substantially below those required for conventionally prepared B.sub.4 C powders. Also, the microstructure of the pressed parts reveals pure, uniform grains, which are required in high strength ceramic parts. Thus this invention also concerns boron carbide having the following characteristics:
(a) B/C ratio of 3.9 to 4.2; PA1 (b) metal impurities of less than 10 ppm per metal; PA1 (c) particle size range of 100 to 1300 .ANG.; PA1 (d) monodispersed powder; PA1 (e) surface area of at least 50 m.sup.2 /g; PA1 (f) microcrystalline structure; and PA1 (g) capable of being densified to theoretical density (2.52 g/cm.sup.3).